Constraining A → Z H with H → t t ¯ in the low-mass region

The decay A → Z H is a characteristic signal of two-Higgs-doublet models (2HDMs), where A and H lie primarily within the same S U ( 2 ) L multiplet, leading to a coupling of order g 2 to the Z boson. The subsequent decay H → t t ( * ) is particularly promising, as it gives rise to distinct final states involving multiple leptons and b -jets. The required splitting between m A and m H can naturally occur near the electroweak scale while being consistent with perturbative unitarity. Whereas dedicated ATLAS and CMS searches focused on the region with both top-quarks on-shell, we cover lower masses where one top quark is off-shell by recasting Standard Model t t ¯ Z measurements of ATLAS and CMS. The obtained limits on σ ( A → Z H ) × Br ( H → t t ¯ ) are between 0.12 pb and 0.62 pb. Interestingly, we observe these stringent limits despite a preference (up to 2.5 σ ) for a nonzero new physics signal, most pronounced around for m A ≈ 450 – 460 GeV and m H ≈ 290 GeV , with a best-fit value of σ ( A → Z H ) × Br ( H → t t ¯ ) ≈ 0.3 pb . This cross section can be accommodated within a top-philic 2HDM for a top-Yukawa coupling of the second Higgs doublet of 0.16 ≲ μ t ≲ 0.33 .